Add the spinach to a food processor and pulse until finely chopped.
Add the sausage, spinach, garlic, egg, breadcrumbs, and Parmigiano-Reggiano to a bowl and mix until just combined.
Use a scoop or spoon to portion out your meatballs and roll gently to form. Place in a cast iron skillet or baking dish. If your meat mixture gets sticky wet your hands when rolling.
Allow the meatballs to bake for 20 minutes.
Carefully remove the pan from the oven and add the marinara.
Let your meatballs cook for an additional 10 minutes.
Top with fresh parsley. Serve as an appetizer, with pasta, polenta, or rice. Enjoy!